The fundamentals of the above mentioned problems are well understood, because the origin of these problems is very similar to the well-known “viewing angle” problem found in Liquid Crystal Displays (LCDs) in general. Because of the very large impact of the viewing angle problem, found in liquid crystal displays (LCDs) of laptop computers and TV, worldwide interest in this problem has led to a number of solutions.
It has to be emphasized that active light filters, based on LCD light shutters are very specialized LCD products typically used for eye protection and stereovision devices. As such they are subject to noticeably different technical requirements, as found with typical LCD display applications. A point to be emphasized here is, that due to the voltage limitations, that exist for picture elements in the multiplex addressed LCD screens of lap-top computers and TV, as well as the overall user requirements for these display panels, the developed technical solutions did not lead to the desired performance and cost target for a shutter device, that has to be used for example in the personal protection devices. In the case of a shutter device for personal eye-protection applications (e.g.: welding glasses, . . . ), the drive voltage can be two to four times that of a LCD for a lap-top computer, and it is much more critical, that very high light attenuations are reached and the off-normal axis light extinction properties of the device are excellent.
The automatic LCD active light filters in their “optically closed” state (—eye protection!)
usually operate in a “dark scale” regime (light attenuation>10), quite often even as two state devices in optically fully open and highly “optically closed state” (—the major issue of the presented patent application). They typically require very high light attenuation, which can be realized only with more than one (2 or even 3) LCD light shutters joint in the final protective device. Having more than one LCD light shutter in a device, significantly affects the overall problem of the angular dependence of the light attenuation as well as its symmetry around the normal to the LCD plane.
The most typical examples of stacked LCDs are found with automatic LCD light switching filters for personal protection applications in welding, where light attenuation exceeding 100,000 is required. So Harsch et al (U.S. Pat. No. 4,039,254) and Reisacher at all (DE 3721751) teach that stacking at least two standard Twist Nematic (TN) LCD (90° twist) light shutters in tandem provides the light attenuation, as required for eye protection in welding applications. None of them however discloses the advantage of the choice of the “complementary TN LCD light shutters” (complementary viewing angles), which can improve and symmetrize the overall viewing angle of the LCD welding filter.
Significantly more detailed analysis is provided in the patents of A. Hornell (WO 97/15254, WO97/15255, and WO95/29428) and in the publication by S. Palmer et al (Appl. Optics, 35, 13, (1996)). They teach that stacking two Low Twist Nematic (<90 degrees) cells (LTN LCD) provides adequate light attenuation. They further teach that the LCD cells and the adjacent crossed polarizers are mutually oriented in such a way that the polarizers are aligned exactly along the bisectrices of the LC alignment directions in both LCD cells—a solution already introduced before by Young et all (U.S. Pat. No. 5,940,155). The authors also teach that a positive-birefringent layer having its optic axis oriented in the plane of the LCD light filter is used to reduce the residual retardation of the LC cell in the optically closed state in order to reduce the driving voltage necessary for achieving high light attenuation.
The use of a different LCD technology (Pi-cell), exhibiting a significantly better angular symmetry than the above mentioned “TN and LTN solutions” is introduced for automatic LCD light filters by Fergason et al (U.S. Pat. No. 5,515,186) from OSD Envision. They teach that stacking at least two Pi-cells in a tandem provides adequate light attenuation. With the difference from the above-mentioned Hornell's patents (LTN technology) the polarizers are not aligned along the bisectrices of the LC alignment directions on the boundaries of the LCD cells. The authors also claim the use of positive-birefringent layers, having their optic axis oriented in the plane of the LCD light filter and perpendicularly to the LC-molecular alignment. Such an additional element is necessary in order to compensate for the residual birefringence of the Pi-cell in the optically closed state so that the driving voltage necessary for achieving the required high light attenuation is reduced to acceptable value. The authors also propose the specific relative orientation of the LCD cells to improve the overall angular dependence of the LCD light switching filter and even introduce the use of additional in-plane (LCD) quarter-wave plate to increase this possibility.
None of the above mentioned patents discloses the use of a negative-birefringent layer with the optic axis oriented along the normal to the LCD light filter plane (negative-birefringent c-plate) to correct for the positive birefringence of the homeotropically aligned LC molecules in the “optically closed” state of the LCD light switching filter—the major source of the annoying high angular dependence of the light attenuation of the LCD light filters. They also do not mention the use of “complementary constructed LC-cells” stacked in a tandem in order to further reduce the angular dependence of the LCD light filters. Finally none of the above mentioned technical solutions mentions nor allows a “specific optimization” (other than standard LCD panel solutions) of the LCD cell parameters (like LC-molecular orientation twist angles, relative orientation of the polarizing films with respect to the LC-molecular alignment, . . . ) to get the required value and slope of the light transmission/voltage characteristics. Since the LCD light filters are typically autonomous, battery powered devices, the performances of the driving electronics are rather limited by the possible choice of batteries. The accuracy of the adjustment of the light attenuation and especially the overall power consumption therefore strongly depend on the light transmission/voltage characteristics of the LCD light filters.